1. Field of the Invention
The present invention relates to a distortionless receiving circuit used for a radio receiver, and more particularly to a distortionless receiving circuit which can suppress the generation of signal distortions based on frequency signal inputs received.
2. Description of the Related Arts
When a comparatively larger reception signal in electric field strength is inputted to a radio receiver, excessive signal voltages will be applied to the internal circuits of the receiver, thereby causing the occurrence of distortions. Distortionless receiving circuits have conventionally been used to suppress the generation of such distortions. The distortionless receiving circuits are typically constructed by a variable attenuator controlled in dependence on the intensity of reception signals. For example, a superheterodyne-type radio receiver utilizing a distortionless receiving circuit is disclosed in Japanese Patent Laid-open No. 194414/83 (JP, A, 58-194414).
FIG. 1 illustrates the radio receiver disclosed in the Laid-open No. 194414/83 document. Located between an antenna 11 and a radio frequency (RF) amplifier 13 is a variable attenuator 12 which comprises a group of resistors arranged in a ladder manner, and is disposed to be controlled using open-collector outputs of an A/D converter 22. A mixer 14 is so connected on the output side of the high frequency amplifier 13 that it can achieve the frequency mixing of the local oscillation output signals from a local oscillator (LO) 15 and the output signals from the radio frequency amplifier 13. On the output side of the mixer 14, there are disposed a bandpass filter 16, an intermediate frequency (IF) amplifier 17, a detector 18, and a low frequency (LF) amplifier 19 in this order, and a speaker 20 is connected to the output terminal of the low frequency amplifier 19. In addition, a carrier rectifier 21 is also disposed in such a manner that it receives the carriers taken out at the detector 18. The output of the carrier rectifier 21 is applied to the A/D converter 22 on one hand, and applied as AGC (Automatic Gain Control) signal to the radio frequency amplifier 13 and the intermediate frequency amplifier 17 on the other hand.
In operation, the reception signals from the antenna 11 pass through the variable attenuator 12 and are then amplified at the radio frequency amplifier 13. The output of the amplifier 13 is subject to frequency conversion at the mixer 14 and then so processed at the bandpass filter 16 that only intermediate frequency components will be extracted. The intermediate frequency components, after having been amplified at the intermediate frequency amplifier 17, are detected at the detector 18 and converted into low frequency signals. Consequently, the speaker 30 is driven with the low frequency signals from the low frequency amplifier 19. In addition, the carrier components taken out at the wave detector 18 are converted into dc signals at the carrier rectifier 21 to operate the A/D converter 22 in compliance with the voltages of dc signal, thereby allowing the amount of attenuation in the variable attenuator 12 to be determined. Moreover, the dc signals are also used to decide the amount of AGC in the radio frequency amplifier 13 and the intermediate frequency amplifier 17. Finally, the gain control for the variable attenuator 12 will be carried out stepwise in compliance with the magnitude of the carrier components in the frequency signals received, and the gain control for the minor variations in the intensity of receiving electric field will be effected through the radio frequency amplifier 13 and the intermediate frequency amplifier 17.
However, this type of radio receiver involves such a problem that NF (Noise Figure) values may be aggravated due to the passage loss through the variable attenuator, thus resulting in the degradation in reception sensitivity.
It is therefore an object of the present invention to provide a distortionless receiving circuit that, in the case of high electric field strength, can prevent the generation of distortions without causing the reception sensitivity in the case of lower electric field strength to be degraded.